Buck Barker assumed the new cataloging responsibilities in the 1950's as Burdick worked to mount his collection for the Met. After the 1960 ACC was released, regular updates coordinated by Barker appeared in the Card Collectors Bulletin, from November 21, 1960 throughFebruary 1972. Around 1968 an attempt was made to change the way newer R cards were catalogued, in keeping with a long tradition of the ACC numbering being tweaked as time passed. No, the numbering is not and never was set in stone.
Chris Benjamin collected all of these updates and issued them as a booklet in 1990, which is a fantastic resource. I would be interested to hear about the other updates that were issued before 1960 as such publications can really help pinpoint certain things. Here is the cover of the collected ACC Updates:
